Jack himself is useless, she needs the protein receptor to be put into the under 25 year olds, and for that she needs high level Starfleet Infiltration.
Also keep in mind that cloak and daggers is an alien (no pun intended) concept to the Borg, their modus operandi is a frontal assault. Furthermore, keep in mind that she is out of resources, she cannot even repair her own cube, the entire thing is falling apart around her. It would not surprise me if she is going all in here.
As for the rogue Changelings. She has to be careful with seeking alliances, if Starfleet gets word of it, it is game over. So, she cannot just openly go searching for allies. For good reason, the Dominion apparently has some intelligent assets infiltrating the rogue Changelings, since Odo was aware that something is going on. Starfleet Intelligence also come close to putting the dots together.